With the recent accelerated reduction in the size and weight of mobile information terminals such as cellar phones, notebook computers, and smartphones, secondary batteries serving as driving power supplies have been required to have a higher capacity. Nonaqueous electrolyte secondary batteries, which are charged and discharged by the movement of lithium ions between positive and negative electrodes, have a high energy density and a high capacity and therefore are widely used as driving power supplies for the above mobile information terminals.
Furthermore, nonaqueous electrolyte secondary batteries have recently attracted attention as power supplies for driving, for example, power tools, electric vehicles (EVs), and hybrid electric vehicles (HEVs, PHEVs) and are promising for various uses. Such power supplies for driving are required to have a higher capacity that allows long-term operation and improved output characteristics in the case where charge and discharge are repeatedly performed with a large current within a relatively short time. In particular, when nonaqueous electrolyte secondary batteries are used as power tools, EVs, HEVs, PHEVs, and the like, a higher capacity needs to be achieved while output characteristics during charge and discharge with a large current are maintained.
For example, PTL 1 below suggests that when a group III element on the periodic table is provided on the surfaces of base particles of a positive electrode active material, the degradation of charge storage characteristics due to a decomposition reaction of an electrolytic solution that occurs at an interface between the positive electrode active material and the electrolytic solution when the charge voltage is increased can be suppressed.
PTL 2 below suggests that an increase in capacity and an improvement in charge-discharge efficiency can be achieved by coating a positive electrode active material containing lithium and at least one of nickel and cobalt with a borate compound and then heat-treating the positive electrode active material.